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vinicius-tersi 7542d364b4 feat: context window monitor hook with agent-side WARNING/CRITICAL alerts

Adds PostToolUse hook that reads context metrics from statusline bridge file and injects alerts into agent conversation when context is low.

Features:
- Two-tier alerts: WARNING (<=35% remaining) and CRITICAL (<=25%)
- Smart debounce: 5 tool uses between warnings, severity escalation bypasses
- Silent fail: never blocks tool execution
- Security: session_id sanitized to prevent path traversal

Ref #212

2026-02-20 14:40:08 -06:00

2.6 KiB

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Context Window Monitor

A PostToolUse hook that warns the agent when context window usage is high.

Problem

The statusline shows context usage to the user, but the agent has no awareness of context limits. When context runs low, the agent continues working until it hits the wall — potentially mid-task with no state saved.

How It Works

The statusline hook writes context metrics to /tmp/claude-ctx-{session_id}.json
After each tool use, the context monitor reads these metrics
When remaining context drops below thresholds, it injects a warning as additionalContext
The agent receives the warning in its conversation and can act accordingly

Thresholds

Level	Remaining	Agent Behavior
Normal	> 35%	No warning
WARNING	<= 35%	Wrap up current task, avoid starting new complex work
CRITICAL	<= 25%	Stop immediately, save state (`/gsd:pause-work`)

Debounce

To avoid spamming the agent with repeated warnings:

First warning always fires immediately
Subsequent warnings require 5 tool uses between them
Severity escalation (WARNING -> CRITICAL) bypasses debounce

Architecture

Statusline Hook (gsd-statusline.js)
    | writes
    v
/tmp/claude-ctx-{session_id}.json
    ^ reads
    |
Context Monitor (gsd-context-monitor.js, PostToolUse)
    | injects
    v
additionalContext -> Agent sees warning

The bridge file is a simple JSON object:

{
  "session_id": "abc123",
  "remaining_percentage": 28.5,
  "used_pct": 71,
  "timestamp": 1708200000
}

Integration with GSD

GSD's /gsd:pause-work command saves execution state. The WARNING message suggests using it. The CRITICAL message instructs immediate state save.

Setup

Both hooks are automatically registered during npx get-shit-done-cc installation:

Statusline (writes bridge file): Registered as statusLine in settings.json
Context Monitor (reads bridge file): Registered as PostToolUse hook in settings.json

Manual registration in ~/.claude/settings.json:

{
  "statusLine": {
    "type": "command",
    "command": "node ~/.claude/hooks/gsd-statusline.js"
  },
  "hooks": {
    "PostToolUse": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "node ~/.claude/hooks/gsd-context-monitor.js"
          }
        ]
      }
    ]
  }
}

Safety

The hook wraps everything in try/catch and exits silently on error
It never blocks tool execution — a broken monitor should not break the agent's workflow
Stale metrics (older than 60s) are ignored
Missing bridge files are handled gracefully (subagents, fresh sessions)

2.6 KiB Raw Blame History